This invention relates generally to an implantable cardiac rhythm management device that may operate in an autothreshold or autocapture verification mode, wherein the rhythm management device is capable of detecting noise that affects signal characteristics of a sensed electrocardiogram signal. The presence of noise in an electrocardiogram signal occurring in, for example, a capture detection window may adversely affect the ability of the device to respond to patient needs.
Regardless of the type of cardiac rhythm management device that is employed, all operate to stimulate excitable heart tissue cells adjacent to the electrode of the lead coupled to the rhythm management device. Response to myocardial stimulation or xe2x80x9ccapturexe2x80x9d is a function of the positive and negative charges found in each myocardial cell within the heart. More specifically, the selective permeability of each myocardial cell works to retain potassium and exclude sodium such that, when the cell is at rest, the concentration of sodium ions outside of the cell membrane is approximately equal to the concentration of potassium ions inside the cell membrane. However, the selective permeability also retains other negative particles within the cell membrane such that the inside of the cell membrane is negatively charged with respect to the outside when the cell is at rest.
When a stimulus is applied tq the cell membrane, the selective permeability of the cell membrane is disturbed and it can no longer block the inflow of sodium ions from outside the cell membrane. The inflow of sodium ions at the stimulation site causes the adjacent portions of the cell membrane to lose its selective permeability, thereby causing a chain reaction across the cell membrane until the cell interior is flooded with sodium ions. This process, referred to as depolarization, causes the myocardial cell to have a net positive charge due to the inflow of sodium ions. The electrical depolarization of the cell interior causes a mechanical contraction or shortening of the myofibrils of the cell membrane. The syncytial structure of the myocardium will cause the depolarization originating in any one cell to radiate through the entire mass of the heart muscle so that all cells are stimulated for effective pumping. Following heart contraction or systole, the selective permeability of the cell membrane returns and sodium is pumped out until the cell is re-polarized with a negative charge within the cell membrane. This causes the cell membrane to relax and return to the fully extended state, referred to as diastole.
In a normal heart, the sino-atrial (SA) node initiates the myocardial stimulation described above. The SA node comprises a bundle of unique cells disposed within the roof of the right atrium. Each cell membrane of the SA node has a characteristic tendency to leak sodium ions gradually over time such that the cell membrane periodically breaks down and allows an inflow of sodium ions, thereby causing the SA node cells to depolarize. The SA node cells are in communication with the surrounding a trial muscle cells such that the depolarization of the SA node cells causes the adjacent a trial muscle cells to depolarize. This results in a trial systole wherein the atria contract to empty and fill blood into the ventricles. The a trial depolarization from the SA node is detected by the atrioventricular (AV) node which, in turn, communicates the depolarization impulse into the ventricles via the Bundle of His and Purkinje fibers following a brief conduction delay.
In this fashion, ventricular systole lags behind atrial systole such that the blood from the ventricles is pumped through the body and lungs after being filled by the atria. Atrial and ventricular diastole follow wherein the myocardium is repolarized and the heart muscle relaxes in preparation for the next cardiac cycle. It is when this system fails or functions abnormally that a cardiac rhythm management device may be needed to deliver an electronic pacing stimulus to the heart so as to maintain proper heart rate and synchronization of the filling and contraction of the a trial and ventricular chambers of the heart.
The success of a cardiac rhythm management device in causing a depolarization or evoking a response hinges on whether the energy of the pacing stimulus as delivered to the myocardium exceeds a threshold value. This threshold value, referred to as the capture threshold, represents the amount of electrical energy required to alter the permeability of the myocardial cells to thereby initiate cell depolarization. If the energy of the pacing stimulus does not exceed the capture threshold, then the permeability of the myocardial cells will not be altered and thus no depolarization will result. If, on the other hand, the energy of the pacing stimulus exceeds the capture threshold, then the permeability of the myocardial cells will be altered such that depolarization will result.
Changes in the capture threshold may be detected by monitoring the efficacy of stimulating pulses at a given energy level. If capture does not occur at a particular stimulation energy level which previously was adequate to effect capture, then it can be surmised that the capture threshold has increased and that the stimulation energy should be increased. On the other hand, if capture occurs consistently at a particular stimulation energy level over a relatively large number of successive stimulation cycles, then it is possible that the capture threshold has decreased such that the stimulation energy is being delivered at a level higher than necessary to effect capture. This can be verified by lowering the stimulation energy level and monitoring for loss of capture at the new energy level.
The ability to detect capture in a cardiac rhythm management device is extremely desirable in that delivering stimulation pulses having energy far in excess of the patient""s capture threshold is wasteful of the cardiac rhythm management device""s limited power supply. In order to minimize current drain on the power supply, it is desirable to automatically adjust the cardiac rhythm management device such that the amount of stimulation energy delivered to the myocardium is maintained at the lowest level that will reliably capture the heart. To accoimplish this, a processes known as xe2x80x9ccapture verificationxe2x80x9d and xe2x80x9cautothresholdxe2x80x9d may be performed wherein; the cardiac rhythm management device monitors to determine whether an evoked response or depolarization occurs in the heart following the delivery of each pacing stimulus pulse. The occurrence of noise during the autothreshold or autocapture mode may lead to an improper conclusion related to capture or non-capture.
Obel et al. in U.S. Pat. No. 5,861,008 apparently describes a heart-simulating device that measures the amplitude of noise and if a predetermined number of noise detections exceeding a predetermined amplitudes (detected at a predetermined frequency) then the device assumes that the noise is significant and adjusts accordingly. In some instances the amplitude of noise may not be sufficient or the frequency may not be enough such that the device described by Obel et al. adjusts accordingly, while in actuality the noise may have significant impact on the accuracy of evoked response determination. Hence, there is a need for a cardiac rhythm management device capable of detecting noise independent of the amplitude of noise or the frequency of occurrence of noise. These and numerous other disadvantages of the prior art necessitates the need for the method and apparatus provided by the present invention.
The present invention provides for a cardiac rhythm management device and method capable of detecting noise, wherein the device is operable in either a unipolar or bipolar sensing mode. In the past, during unipolar sensing, the presence of noise has adversely affected the ability of conventional rhythm management devices to accurately identify an evoked response to a stimulation pulse. The device of the present invention provides for unipolar sensing during either an autothreshold or capture verification mode, wherein the presence of noise affecting the signal characteristics of an electrocardiogram signal is identified during either mode. The cardiac rhythm management device may be electrically coupled to one or more known suitable leads having pacing/sensing electrodes coupled thereto.
Without limitation, the cardiac rhythm management device of the present invention includes a power supply, pulse generator that generates stimulation pulses, and controller coupled to pacing/sensing leads to receive sensed electrocardiogram signals (IEGM signals). The controller is electrically coupled to the pulse generator and controls delivery of the stimulation pulses to the heart. The controller also controls detection of intrinsic and evoked responses and may operate in an autocapture or autothreshold mode. A noise detection circuit of the controller allows for detection of noise affecting the signal characteristics of the IEGM signal.
The controller may be in any of several forms including a dedicated state device or a microprocessor with code, and may include ROM memory for storing programs to be executed by the controller and RAM memory for storing operands used in carrying out the computations by the controller. Those skilled in the art will appreciate that stimulation circuitry, sensing circuitry, timing circuitry, and wave detection circuitry among others may all be included within the controller. The controller and components contained therein or coupled thereto detect and distinguish cardiac depolarization deflections and noise deflections from the electrocardiogram signal. A peak detector, for example, may be utilized to determine the amplitudes of the cardiac depolarization deflections and artifact deflections.
Following delivery of a stimulation pulse, noise affecting signal characteristics of the electrocardiogram signal is assumed or determined if a minimum peak timing (Min T or tmin) is detected occurring during a predetermined period of time following delivery of a stimulation pulse. The predetermined period of time begins after a predetermined refractory period and more specifically may occur within an evoked response detection window controlled by the controller. Without limitation, in the preferred embodiment the refractory period ranges between 3-15 msec. and the predetermined noise detection period of time following the pace is between 20-55 msec. in length.
In use, the controller controls delivery of a stimulation to a patient""s heart. An electrocardiogram signal from the patient""s heart is sensed and the controller determines the time at which a minimum peak occurs following delivery of the stimulation pulse. If the time of the minimum peak occurs during a predetermined period of time following delivery of the stimulation pulse, then noise affecting the electrocardiogram signal is determined or assumed. During autocapture verification or autothreshold, the predetermined period of time is set to begin after expiration of a predetermined refractory period. Further, the predetermined period of time may be set to occur within an evoked response detection window controlled by the controller. During a capture verification mode, a backup stimulation is delivered and the capture verification stimulation mode is abandoned if noise is detected. During the autothreshold stimulation mode, a current stimulation amplitude of the stimulation pulse is maintained during the autothreshold stimulation mode if noise is detected. The autothreshold stimulation mode is terminated if detection of noise persists.